(a) Field of the Invention
This invention relates to an image forming apparatus such as a printer, a photocopier, and a facsimile machine and, more particularly, to an apparatus utilizing an intermediate transfer body.
(b) Description of Related Art
As image forming apparatuses of a full-color electrophotographic method, an apparatus has been known in which plural toner images are sequentially formed on a single photosensitive body and are overlapped on an intermediate transfer body to form full color images. More specifically, the apparatus is described with reference to FIG. 5. Developing devices 4a, 4b, 4c, 4d having respective toners of different colors are sequentially arranged as to face to an image carrying means 1 as a single photosensitive drum, thereby sequentially forming the toner images of the respective colors. The toner images of the respective colors are sequentially overlapped onto the intermediate transfer body 61 from the image carrying means 1, and the toner images overlapped on the intermediate transfer body 61 are transferred onto a transfer material P with a secondary transfer roller 66. Then, the transfer material carrying the toner images is made through a fixing device 8, and thereby the image forming apparatus is practically used for obtaining permanently fixed images upon application of heat and pressure at the fixing device. Where four color toners are used to form full-color images, the apparatus has a structure that the toner image is overlapped four times on the intermediate transfer body to form images. This apparatus is of, so-called, a four-pass system.
The image forming apparatus described above necessarily rotates the intermediate transfer body for a number of times equal to the number of the colors of the overlapping toners to overlap the toner images, so that the productivity is not so high. As an image forming apparatus whose productivity is improved and whose size is not significantly large, an image forming apparatus exists in which plural photosensitive bodies are provided and in which two rotations of the intermediate transfer body make the toner images of the desired color number overlapped in the intermediate transfer body. This apparatus is of, a so-called, two-pass system. Where, for example, images with overlapped four-color toners are formed, the image forming apparatus forms the toner images in which the four-color toners are overlapped by two rotations of the intermediate transfer body where formed with two photosensitive bodies and where the toner images of two-colors are carried on the intermediate transfer body at each rotation of the intermediate transfer body. That is, in this case, the apparatus ensures advantageously productivity twice to that of the image forming apparatus having a single photosensitive body.
With this apparatus, however, image formation requires two rotations of the intermediate transfer body, and the following problems may be raised where images are successively formed on plural sheets. There may raise a problem that both transfer operations are interfering with each other where the fourth color toner image of a previous image is transferred from one photosensitive body to an intermediate transfer body and at the same time the first color toner image of the subsequent image is transferred from the other photosensitive body to the intermediate transfer body. That is, the transfer operation of one body may affect the transfer operation of the other body. With this interference at the transfer operation, transfer defects may occur.